WIDELY DIVERGENT BEAMS OF FAST PARTICLES GENERATED BY COSMIC RADIATION

Oppenheimer and Ney1 in 1949 used balloons to lift a Wilson chamber to an altitude of about 25 km (pressure 14 g/cm²) at 55° N latitude. Several lead plates, 6 mm thick, were placed in the chamber. Among the many photographs obtained during these ascents, there are several dozen in which, to the authors’ knowledge, one or another phenomenon is recorded that is of considerable interest. The most characteristic photograph is shown in the figure. From the lead plate (third from the top) emerges a widely divergent beam of particles (about 30), possessing minimum (relativistic) ionization. All these particles are completely absorbed in the next lead plate. The unusual nature of such photographs consists in the following. If we assume that the particles emerging from the lead are electrons, then in this case, even if all these electrons have not very large energies, on the basis of shower theory it seems improbable that such a large shower could be completely absorbed in a six-millimeter lead plate. Meanwhile, only in rare cases did one or two particles from such showers pass into the lower lead plates. These shower particles obviously also cannot be any of the known types of mesons: relativistically ionizing mesons have great penetrating power and would pass through all the lead plates. Further, it is obvious that these weakly ionizing and weakly penetrating particles cannot be protons or heavier particles either. Since the range of these particles is less than 6 mm of lead and they apparently do not radiate, from this one can estimate an upper limit for their mass, which turns out to be equal to twenty electron masses. A new series of similar photographs was obtained in the work of Rau and Harris2, who raised a Wilson chamber to altitudes between 21 and 27 km at 55° N latitude. They recorded showers containing from 30 to 100 particles diverging widely from a lead plate, and almost all these particles ionize weakly. The total energy of such showers is estimated at several hundred million volts. Unfortunately, the presence of a large background in the Wilson chamber did not make it possible to clarify the nature of the particles generating such showers. The authors believe that they are produced both by ionizing and by non-ionizing radiation. Analysis of the photographs shows that, in the case of small showers, the beginnings of all the observed trajectories converge at a common point located in the lower part

lead plate. In the case of showers consisting of a larger number of particles, the beginning of the tracks is contained in a small volume. Thus, the available data do not make it possible to determine whether these showers originate in a single event or in a sequence of events occurring one after another.

A. V.

References

1. F. Oppenheimer and E. P. Ney, Phys. Rev. 76, 1418 (1949).
2. R. R. Rau and G. G. Harris, Phys. Rev. 79, 915 (1950).